This invention relates to glass optical waveguides such as optical fibers and planar waveguides and, in particular, to such waveguides passivated against hydrogen-induced loss increases.
While optical waveguides, such as optical fibers, can carry optical signals over remarkably long distances between repeater stations, it has been observed that such fibers suffer long-term increases in loss, particularly for transmission of light in the 1.55 and 1.31 micrometer wavelength regions. One type of loss increase is attributed to the reaction of hydrogen with defects in GeO2-doped waveguides containing alkali impurities such as Na and Li. Another type occurs when H2 reacts at defect sites in undoped silica areas. The rate of loss increase of the latter type can be reduced by treating the fiber with hydrogen during the draw process (U.S. Pat. No. 5,059,229 issued to M. G. Blankenship et al. on Oct. 22, 1991). But this rate reduction is obtained only at the cost of increased loss at the outset. Such increased loss is undesirable for many high performance applications such as long distance transmission.
Hydrogen-induced loss is particularly a problem in erbium-doped (Er-doped) amplifier fibers. Accelerated hydrogen aging tests have shown that Er-doped amplifier fibers react quickly with even trace levels of H2, causing spectrally broad loss increases that can influence signal and pump wavelengths. See P. J. Lemaire et al., Prediction of Long-Term Hydrogen-Induced Loss Increases in Er-Doped Amplifier Fibers, 5 IEEE Photonics Technology Letters 214 (Febrary 1993) and P. J. Lemaire et al., Hydrogen-induced loss increases in erbium-doped amplifier fibers: Revised Predictions OFC Technical Digest Feb. 20-25 (1994), Paper FF1. U.S. Pat. No. 4,685,945 to Friedemann Freund describes a method of enhancing long-term stability in high purity vitreous silica optical fiber by permeating deuterium molecules into the fiber and then propagating through the fiber high intensity light with a wavelength less than about 650 manometers. The difficulty with this process is most useful fibers absorb heavily at such short wavelengths, making uniform treatment difficult. Accordingly, there is a need for improved glass waveguides passivated against hydrogen-induced loss increases.
Applicants have discovered that glass optical waveguides subject to hydrogen-induced loss increases can be passivated by treating the glass with deuterium. The deuterium-treated glass not only exhibits a lower rate of loss increase when later exposed to environments containing H2, but also retains high transmission of light in the 1.55 and 1.31 micrometer wavelength regions immediately after the deuterium heat treatment. The method applies to Er-doped fiber, transmission fiber and planar waveguides. Under some circumstances, hydrogen can be substituted for deuterium.